The present invention relates to a hydraulic anti-skid apparatus for installation in a vehicle braking system between a master cylinder and a wheel brake cylinder to prevent the road wheels of the vehicle from locking in braking operation.
As one of such anti-skid apparatuses as described above, there has been proposed an anti-skid apparatus which comprises a reservoir arranged to store an amount of hydraulic fluid, a hydraulic power pressure source connected to the reservoir to produce a hydraulic power pressure higher than a braking pressure applied to a wheel brake cylinder from a master cylinder, an accumulator connected to the pressure source to store the hydraulic power pressure, a cut-off valve disposed within a braking circuit connecting the master cylinder to the wheel brake cylinder, the cut-off valve including a valve seat arranged to permit the fluid communication between the master cylinder and the wheel brake cylinder, a valve element arranged to cooperate with the valve seat to block the fluid communication between the master cylinder and the wheel brake cylinder when moved to a closed position, and a piston arranged to be applied at the opposite ends thereof with the braking pressure from the master cylinder and the hydraulic power pressure from the accumulator and being associated with the valve element to maintain the valve element in an open position while the power pressure is applied thereto and to be displaced by the braking pressure when released from the power pressure to effect movement of the valve element to the closed position and subsequently to increase a capacity downstream of the cut-off valve, and a changeover valve disposed within a power pressure circuit connecting the accumulator to the cut-off valve to normally permit the hydraulic power pressure applied to the piston therethrough and to connect the piston to the reservoir when the road wheels of the vehicle tend to be locked in braking operation. In the above-described anti-skid apparatus, the hydraulic power pressure acting on the piston is exhausted only when the road wheels of the vehicle tend to be locked. This means that the hydraulic power pressure is always applied to the cut-off valve even when the master cylinder is inoperative. This causes a hydraulic heavy load acting on sealing members assembled within the cut-off valve, resulting in a decrease in the life-span of the sealing members.